From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.3 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 6EDD0C43387 for ; Mon, 7 Jan 2019 15:54:44 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 398512173C for ; Mon, 7 Jan 2019 15:54:44 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="JLSAJM2Y" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730320AbfAGPyn (ORCPT ); Mon, 7 Jan 2019 10:54:43 -0500 Received: from bombadil.infradead.org ([198.137.202.133]:56652 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726511AbfAGPym (ORCPT ); Mon, 7 Jan 2019 10:54:42 -0500 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20170209; h=In-Reply-To:Content-Type:MIME-Version :References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=EB8jJo74ngJGhrjSVclXwp46WwssCxFPErndM29eks4=; b=JLSAJM2YVQOUjWUV138MvuYR9 3dPjf9waKIDM2/SO2xRhDIZubSfm4WiTjU1KRVlE61YjDLJBX6BUQeR6hXY70aeXEitHFolg5nUWF BaJYfMxGBI5kEhXiGbsj4uGDUZOO0cKwQ5TgztVLIm0k1/Hvw9xclye+ufU0bEAbJ6UEANhk8ffkc h0Rd/oZ/ZVFxuhY1jj5oXhfWd4feL+hb9fzMcpe5C8RYWl/rF4Cemdu+QVbcrvCfOzQAKAU9eXjfG A7P6JZ5ObfZZxoErjKxkU3AM7TO+uz8iQYnhPcQg5ui+LYFSbIOHWiMF42+c9mZkfoYcQ8U43sJO7 kecSpl8eQ==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=hirez.programming.kicks-ass.net) by bombadil.infradead.org with esmtpsa (Exim 4.90_1 #2 (Red Hat Linux)) id 1ggXEP-0006cV-MG; Mon, 07 Jan 2019 15:54:26 +0000 Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 3023020074A04; Mon, 7 Jan 2019 16:54:23 +0100 (CET) Date: Mon, 7 Jan 2019 16:54:23 +0100 From: Peter Zijlstra To: "Michael S. Tsirkin" Cc: Jason Wang , linux-kernel@vger.kernel.org, Alan Stern , Andrea Parri , Will Deacon , Boqun Feng , Nicholas Piggin , David Howells , Jade Alglave , Luc Maranget , "Paul E. McKenney" , Akira Yokosawa , Daniel Lustig , linux-arch@vger.kernel.org, netdev@vger.kernel.org, virtualization@lists.linux-foundation.org, Jonathan Corbet , Richard Henderson , Ivan Kokshaysky , Matt Turner , Arnd Bergmann , Luc Van Oostenryck , linux-doc@vger.kernel.org, linux-alpha@vger.kernel.org, linux-sparse@vger.kernel.org Subject: Re: [PATCH RFC 3/4] barriers: convert a control to a data dependency Message-ID: <20190107155423.GM2218@hirez.programming.kicks-ass.net> References: <20190102205715.14054-1-mst@redhat.com> <20190102205715.14054-4-mst@redhat.com> <86023cbe-d1ae-a0d6-7b75-26556f1a0c1f@redhat.com> <20190106231756-mutt-send-email-mst@kernel.org> <20190107094610.GA2861@worktop.programming.kicks-ass.net> <20190107082223-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190107082223-mutt-send-email-mst@kernel.org> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jan 07, 2019 at 08:36:36AM -0500, Michael S. Tsirkin wrote: > On Mon, Jan 07, 2019 at 10:46:10AM +0100, Peter Zijlstra wrote: > > How about naming the thing: dependent_ptr() ? That is without any (r)mb > > implications at all. The address dependency is strictly weaker than an > > rmb in that it will only order the two loads in qestion and not, like > > rmb, any prior to any later load. > > So I'm fine with this as it's enough for virtio, but I would like to point out two things: > > 1. E.g. on x86 both SMP and DMA variants can be NOPs but > the madatory one can't, so assuming we do not want > it to be stronger than rmp then either we want > smp_dependent_ptr(), dma_dependent_ptr(), dependent_ptr() > or we just will specify that dependent_ptr() works for > both DMA and SMP. The latter; the construct simply generates dependent loads. It is up to the CPU as to what all that works for. > 2. Down the road, someone might want to order a store after a load. > Address dependency does that for us too. Assuming we make > dependent_ptr a NOP on x86, we will want an mb variant > which isn't a NOP on x86. Will we want to rename > dependent_ptr to dependent_ptr_rmb at that point? Not sure; what is the actual overhead of the construct on x86 vs the NOP?